More than a century of research shows that increasing the gap between study episodes using the same material can enhance retention, yet little is known about how this so-called distributed practice effect unfolds over nontrivial periods. In two three-session laboratory studies, we examined the effects of gap on retention of foreign vocabulary, facts, and names of visual objects, with test delays up to 6 months. An optimal gap improved final recall by up to 150%. Both studies demonstrated non-monotonic gap effects: Increases in gap caused test accuracy to initially sharply increase and then gradually decline. These results provide new constraints on theories of spacing and confirm the importance of cumulative reviews to promote retention over meaningful time periods. (Contains 4 footnotes, 5 figures and 3 tables.) [This article was printed in the in-press version of: "Experimental Psychology" (2009). Its contents may differ from the final published version.]